Hitherto, in the capacitor of this kind a safety vented structure is necessary, since when an excessively large current flows as a result of a circuit trouble the capacitor element is liable to be heated and a large amount of gas is produced, thereby to increase pressure in the case to generalte a danger of explosion.
As the safety vented structure, generally is adopted such a measure that a safety vent of decreased thickness is provided on sealing member of a sealed case or the case as such, so that, when gas pressure in the case reaches a value immediately before explosion, the safety vent is operated by the gas pressure to discharge the gas, thereby to prevent the explosion before its happening.
FIG. 1 shows representative structure of the sealed type electrolytic capacitor. In the figure, numeral 1 designates a capacitor element soaked with electrolyte, numeral 2 designates a cylindrical case made of metal for enclosing the capacitor element 1, numeral 3 designates a sealing member for sealing an opening part of the case 2, numeral 4 designates terminals fixed to the sealing member 3. The capacitor element 1 is constituted by winding anode foil having anodic oxide film and a cathode foil with electrolytic paper inbetween, and a part designated by dotted lines is winding center part. A safety vent 6 of decreased thickness is provided at the bottom part of the case 2.
As to this safety vent, hitherto various kinds are invented or devised and reduced into practice. FIG. 2A through 2C show one example thereof, and FIG. 2A is a case which provides +* shaped safety groove 6a which crosses at center P of a bottom face of a case 2, FIG. 2B is a case which provides safety groove 6b of V letter shape, and FIG. 2C is a case which provides a ring-shaped safety groove 6c.
Hereupon, when an excessively large current flows as a result of circuit trouble, the capacitor element 1 generates heat, and specially the temperature at the winding center part 5 becomes 10.degree.-20.degree. C. higher than peripheral part. Accordingly, when the safety vent 6 provided at the bottom of the case operates, gas generated in the case is discharged passing almost through the winding center part 5. Since this winding center part 5 has unstable beginning end from which electrode foil and electrolytic paper constituting the capacitor element are started to be wound, it is liable to be deformed, and by gas discharging at operation of the safety vent 6 the winding center part 5 is deformed to be pulled outside. Accordingly, when the safety groove 6a, 6b and 6c operate the capacitor element 1, a part of the winding center part 5 is pulled out as shown in FIGS. 3A, 3B and 3C, thereby to make short-circuiting of anode and cathode of the capacitor element 1, and such a short-circuit accident is generated that a circuit using the electrolytic capacitor is short-circuited thereby to destroy other components, too.